1. Field of the Invention
The present invention relates generally to the formation of cubic boron nitride (c-BN) films on substrates such as cutting tools. More particularly, the present invention relates to the formation of such films at low pressure and relatively low temperature.
2. Description of Related Art
C-BN is a very important technological material. It has properties similar to those of diamond: it is chemically inert, thermally highly conducting, and electrically insulating. The material is also known to be the hardest material after diamond. Consequently, c-BN has found uses in many industrial applications ranging from high precision cutting and grinding of ferrous materials to high temperature electronic devices.
Boron nitride is known to have two crystallographic structures in addition to c-BN. These two structures are tetragonal boron nitride (t-BN) and hexagonal boron nitride (h-BN). C-BN is much harder than either of the two alternate crystal structures and therefore is the preferred coating material for ultra hard tools. At present, cubic boron nitride is deposited on various substrates using high pressure (approximately 45-55 kbar) and high temperature (1400.degree.-1700.degree. C.) conditions. In order for c-BN to be used in a wide variety of applications, it is necessary that processes be developed wherein thin films of c-BN can be formed onto a variety of substrates without requiring high pressures and high temperatures.
Attempts to deposit c-BN films at low pressures have not been successful. For example, present low pressure methods have only been able to produce films containing up to 10% c-BN. Accordingly, there is a present need to develop simple and efficient methods for depositing films of cubic boron nitride onto a wide variety of substrates at low pressure and relatively low temperature.
Diamond has also been widely used as a coating material for ultra hard tools and abrasives. Methods have been developed for growing diamond films at reduced temperatures and pressures. However, diamond films are not well-suited for cutting or abrading iron. Diamond is reactive with iron at the high temperatures generated during cutting operations. Cubic boron nitride is especially well-suited as a substitute for diamond in such cutting operations since cubic boron nitride is not attacked by iron. Accordingly, it would be desirable to provide a method for coating tool bits made from tungsten carbide and other hard materials with cubic boron nitride to provide ultra hard tools which are suitable for use in cutting iron.